In recent years, a digital copying machine and a digital camera need to transmit a digital signal having a large capacity at high speed, in order to achieve the enhancement of the speed and the resolution. When a high-speed digital signal transmits on a printed wiring board, an electromagnetic wave interference (EMI: Electro Magnetic Interference) occurs while a cable and the like which are connected to the printed wiring board work as an antenna, and may affect an operation of other electronic equipment. Because of this, it is necessary to reduce a radiation noise which originates in the high-speed digital signal.
It is known to use an LC resonance type filter as one reduction unit, which removes an unnecessary harmonic component in the digital signal, which causes the radiation noise. The LC resonance type filter uses a series resonance of an inductance and capacitance or a parallel resonance thereof, makes the resonance frequency of the filter coincide with the unnecessary harmonic component, thereby interrupts the unnecessary harmonic component of the digital signal, and suppresses the spread thereof.
In this LC resonance type filter, when the digital signal becomes to transmit at high speed, the unnecessary harmonic component also becomes a higher frequency, and accordingly the resonance frequency of the LC resonance circuit also needs to be set at high frequency. However, if it is intended to form the high-frequency resonance circuit out of a commercial component, the value of the component element becomes an extremely small value, and it becomes difficult to obtain a desired resonance frequency with a standard component.
For such a problem, a method has been known which forms the inductance and the capacitance by a wiring pattern on the printed wiring board and thereby obtains a desired resonance frequency. Japanese Patent Application Laid-Open No. H07-142871 describes a method of using an inductance which occurs in a slim wiring pattern and a via, and/or a capacitance which is formed in planar wiring patterns opposing to each other. The patent describes that a desired resonance frequency is obtained by designing a wiring pattern of the printed wiring board so as to have a shape from which desired inductance and capacitance are obtained.
However, there exist a plurality of unnecessary harmonic components which the digital signal has, and it has been necessary for the printed wiring board in Japanese Patent Application Laid-Open No. H07-142871 to prepare an LC type resonant filter that is formed of sheet wiring patterns for every harmonic component, each of which opposes to a via. In recent years, a miniaturization of a product progresses, and restrictions of arrangement, wiring and the like of a mounted component such as IC increase more and more. In such a situation, it has become a difficult situation to provide such a large area for measures against a radiation noise as to have a plurality of LC resonant filters aligned therein.
Thus, an object of the present invention is to reduce an exclusive region of a resonance portion which can interrupt a plurality of harmonic components, and to reduce EMI originating in a digital signal.